Ancus Martius, the fourth king of Rome, was the grandson of the second Roman king, Numa Pompilius. Legend credits him with building a bridge on wooden piles across the Tiber River, the Pons Sublicius, the first bridge across the Tiber. It is often claimed that Ancus Martius founded the port of Ostia at the mouth of the Tiber River. Cary and Scullard say this is unlikely, but he probably extended Roman territory and gained control of the salt-pans on the south side of the river by Ostia. Cary and Scullard also doubt the legend that Ancus Martius incorporated the Janiculum Hill into Rome, but do not doubt that he established a bridgehead upon it.
Ancus Martius is also thought to have waged war on other Latin cities.
Cary and Scullard: A History of Rome
T.J. Cornell: The Beginnings of Rome.